The present invention relates generally to inkjet printing mechanisms, and more particularly to a composite wiper for removing ink residue from an inkjet printhead.
Inkjet printing mechanisms use cartridges, often called xe2x80x9cpens,xe2x80x9d which eject drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as xe2x80x9cspitting,xe2x80x9d with the waste ink being collected in a xe2x80x9cspittoonxe2x80x9d reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast and light-fast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media.
As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an xe2x80x9coff-axisxe2x80x9d printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an xe2x80x9coff-axisxe2x80x9d stationary reservoir placed at a remote, stationary location within the printer. Since these permanent or semi-permanent printheads carry only a small ink supply, they may be physically narrower than their predecessors, the replaceable cartridges. Narrower printheads lead to a narrower printing mechanism, which has a smaller xe2x80x9cfootprint,xe2x80x9d so less desktop space is needed to house the printing mechanism during use. Narrower printheads are usually smaller and lighter, so smaller carriages, bearings, and drive motors may be used, leading to a more economical printing unit for consumers.
There are a variety of advantages associated with these off-axis printing systems, but the permanent or semi-permanent nature of the printheads requires special considerations for servicing, particularly when wiping ink residue from the printheads. In the past, the printhead wipers have been a single or dual wiper blade made of an elastomeric or plastic material. One earlier wiper blade assembly had a composite wiper, with one side being of elastomeric material, and the other side being of an absorbent fabric material, similar to a felt. This composite absorbent wiper blade had basically a sandwiched structure, using a bonding agent to join the felt pad to one side surface of an otherwise conventional elastomeric wiper blade. Presumably, the felt half of the wiper blade was used to absorb liquid ink residue. Then through capillary forces, the felt material pulled the liquid residue away from the wiper tip.
In past printhead wiping routines, typically the printhead was translated across the wiper in a direction parallel to the scan axis of the printhead. In one printer, the wipers were rotated about an axis perpendicular to the printhead scan axis to wipe. Today, most inkjet pens have nozzles aligned in two linear arrays which run perpendicular to the scanning axis. Using these earlier wiping methods, first one row of nozzles was wiped and then the other row of nozzles was wiped. While these earlier wiping methods proved satisfactory for many traditional dye based inks, unfortunately, they were unacceptable for many newer fast drying pigment inks.
One suitable service station design for pigment-based inks was a rotary device first sold in the DeskJet(copyright) 850C and 855C color inkjet printers, and later in the DeskJet(copyright) 820C and 870C color inkjet printers by Hewlett-Packard Company of Palo Alto, Calif., the present assignee. This rotary device mounted the wipers, primers and caps on a motor-operated tumbler. These pens were wiped using an orthogonal wiping technique, where the wipers ran along the length of the linear nozzle arrays, wicking ink along the arrays from one nozzle to the next to serve as a solvent to break down ink residue accumulated on the nozzle plate. A camming device moved a horizontal arm carrying a wiper scraper into position to clean ink residue from the wipers as they rotated past. The scraper arm had capillary channels formed along the under surface from the scraper tip to an absorbent blotter pad.
A translational or sliding orthogonal wiping system was first sold by the Hewlett-Packard Company in the DeskJet(copyright) 720C and 722C color inkjet printers. The wipers were slid under a stationary horizontal, rigid plastic wiper bar to clean off any clinging ink residue. This wiper bar had an inverted T-shaped head which assisted in scraping the wipers clean. Another wiper system using rotational and vertical motion was first sold by the Hewlett-Packard Company in the DeskJet(copyright) 2000C Professional Series color inkjet printer. This was one of the first service station systems in a Hewlett-Packard Company inkjet printer to use an ink solvent, specifically polyethylene glycol (xe2x80x9cPEGxe2x80x9d), to clean and lubricate the printheads. This service station required two costly motors to operate the service station for moving the service station servicing components both vertically and rotationally.
Most earlier wipers were typically made of a single elastomeric or plastic material, which required comprises to be made in the flexibility of the wiper versus the wiper tip characteristics. Since the entire wiper structure was made of the same material, tailoring the material characteristics of the wiper tip, which contacts the printhead, was impossible without comprising on the flexibility of the blade.
Wiper blade flexibility is important for several reasons. For instance, the flexibility of the wiper blade determines the amount of normal force applied to the orifice plate. Here, the term xe2x80x9cnormal forcexe2x80x9d does not mean the regular amount of force, but rather the engineering term indicating the force which is applied in a direction that is perpendicular to the plane of the orifice plate surface. The amount of normal force applied to the orifice plate affects the wiping capability of the wipers. Too large of a normal force may damage the orifice plate. Indeed, if this normal force is too large, in some printers the printheads may become unseated from their carriage alignment datums. Tangential or shear forces in the wiping direction are also important to assure adequate cleaning of the printhead, and these forces are also affected by the degree of wiper bending.
The flexibility of the wipers is also used to accommodate tolerance stack in the service station components, the printer, the inkjet printhead, and the printhead carriage. The term xe2x80x9ctolerance stackxe2x80x9d refers to the accumulation of the various manufacturing tolerances of components used to construct a particular inkjet printer, where some components may be at the maximum permissible size, while others are at the minimum permissible size. Regardless of which components are used to construct a particular printer, it must function for both extremes of the tolerance stack spectrum, and the wiper blades have been selected to accommodate the tolerance stack variations between the printhead orifice plate and the service station wiper sled. For spacings between the orifice plate and the base of the wiper which are near the smallest extreme, the wiper needs to flex more, and for spacings which are near the largest extreme, the wiper flexes less. From the extreme of maximum flexion to the extreme of minimum flexion, the wiper must adequately clean the printhead without damaging the printhead. Thus, flexibility is a very important factor in wiper design.
Wiper tip design is also a very important factor in wiper design, because the tip is the portion of the wiper which actually contacts the printhead. Besides tip geometry, selection of the tip material is a critical design criteria. For instance, granularity in the wiper tip material may actually scratch the printhead over time, and or damage the nozzle orifices. A damaged nozzle orifice may cause the nozzles to misdirect ink droplets, or expel ink droplets of an inappropriate drop weight, with both of these consequences impacting print quality. Furthermore, selection of the wiper tip material impacts the friction between the wiper and the orifice plate. Over time, excessive friction can also damage the orifice plate. Moreover, this friction may cause the wiper to wear, shortening the life of the wiper and the service station. Another unfortunate consequence of wiper wear is the possibility of forcing torn pieces of the wiper into a nozzle, causing permanent nozzle damage.
Thus, a need exists for a new wiper structure, which can be tailored to meet the competing needs of blade flexibility, and wiper tip characteristics.
According to one aspect of the present invention, a composite wiper is provided for cleaning ink residue from an inkjet printhead in an inkjet printing mechanism. The composite wiper has a wiper blade with a first portion of a first elastomeric material, and a second portion of a second elastomeric material different from the first elastomeric material. In an illustrated embodiment, the wiper blade further includes a third portion of a third elastomeric material different from the first elastomeric material and different from the second elastomeric material.
According to a further aspect of the present invention, an inkjet printing mechanism may be provided with a composite wiper as described above.
An overall goal of the present invention is to provide an inkjet printing mechanism which prints sharp vivid images over the life of the printhead and the printing mechanism, particularly when using fast drying pigment or dye-based inks, and preferably when dispensed from an off-axis system.
Another goal of the present invention is to provide a wiping system for cleaning printheads in an inkjet printing mechanism to prolong printhead life.
Still another goal of the present invention is to provide a printhead wiping system for cleaning printheads in an inkjet printing mechanism, with the system having fewer parts that are easier to manufacture than earlier systems, and which thus provides consumers with a reliable, economical inkjet printing unit.